The present invention relates to the technical field of thermally highly stressed members, such as for example components (pistons or the like) of internal combustion engines, gas turbine blades, combustion chambers etc.
Thermally highly stressed components from the high-temperature region of gas turbines, such as for example blades or vanes, are coated for two reasons:
to protect the blade or vane material from corrosive attacks, and
to reduce the temperatures of the metal to a level, which can be withstood.
Usually, two coatings are applied to the base material. The first one is known as the xe2x80x9coverlayxe2x80x9d coating, which protects against corrosion. The second coating, which is usually referred to as the thermal barrier coating (TBC) and is only applied if need be, serves as the aforementioned thermal isolation.
However, components coated in such a way may also become defective for various reasons: mismatches with respect to the coefficients of thermal expansion between the various materials of which the member consists can cause thermal stresses and deformations in the system. Furthermore, instances of scaling due to oxidation and growth thereof at relatively high temperatures can produce additional stress loads.
These stresses and deformations can finally lead to cracking and spalling of the coating layers. According to FIG. 5, the coating system 10 is therefore usually made up of four different layers:
the base material 11, which is usually several mm thick,
the bond coat 12, which has a thickness of about 0.1 . . . 0.3 mm,
a thermally grown oxide (TGO) layer 13, which grows to a thickness of 0.02 mm, and
the thermal barrier coating 14, which is approximately 0.2 . . . 0.4 mm thick.
In-depth investigations of the stress/deformation behavior have already been carried out on the basis of finite-element networks, in which all four elements of the coating system were modeled in all details, including a nonlinear material behavior. It has been found in these investigations that both the thermal growth of the oxide layers and the creep behavior of the bond coats play a major part in the formation of defects (see in this respect, for example, the article by Freborg, A. M., Ferguson, B. L.: xe2x80x98Modelling oxidation induced stresses in thermal barrier coatingsxe2x80x99. Material Science and Engineering A245, 1998, pages 182-190). However, the results of these investigations cannot be used for predicting lifetimes.
It is therefore the object of the invention to specify a simplified method of estimating the lifetime of a thermal barrier coating, which also takes into account the part played by the changing oxide layer.
The object is achieved by the entirety of the features of claim 1. The essence of the invention is to use in the calculation of the number Ni of cycles to failure material parameters C(xcex4ox) and m(xcex4ox) which depend on the thickness (xcex4ox) of an oxide layer, which is located between the thermal barrier coating 14 and the bond coat 12 and grows with cyclical loading.
The calculation is particularly simple here if, according to a preferred refinement of the method according to the invention, the dependence of the material parameters C(xcex4ox) and m(xcex4ox) on the thickness (xcex4ox) of the oxide layer is assumed to be linear, if furthermore a growth law of the form
xcex4ox=kptxe2x80x3
with a growth constant kp and an exponent n is used for the increase in the thickness (xcex4ox) of the oxide layer with time t, if a damage increment xcex94D, which satisfies the approximation formula       Δ    ⁢          xe2x80x83        ⁢          D      ⁢              (        N        )              ≈            1              C        ⁢                  (          N          )                      ⁢                  (                  Δσ          n                )                    -                  m          ⁢                      (            N            )                              
is calculated, N giving the number of loading cycles, and C(N) and m(N) being parameters which satisfy the equations
C(N)=xcex1c(NT)xe2x80x3+xcex2c
and
m(N)=xcex1m(NT)xe2x80x3+xcex2m
with the exponent n, the constants xcex1c, xcex1m, xcex2c, xcex2m, and the holding time T at high temperature per loading cycle, and if the number of loading cycles to failure Ni of the member is determined by the damage increment being summed up in accordance with the formula   D  =            ∑              N        =        1                    N        i              ⁢          Δ      ⁢              xe2x80x83            ⁢              D        ⁢                  (          N          )                    
until D has reached the value 1.
Further embodiments are disclosed in the dependent claims.